Thermite is a type of pyrotechnic composition of a metal and a metal oxide which produces a highly exothermic reaction, known as a thermite reaction. Thermite reactions have been of interest since the introduction of the Goldschmidt reaction, patented in 1895, between aluminum and iron oxide for the welding of railroad tracks. Other thermite reactions, such as between aluminum and copper oxide illustrated in the equation below, are of interest as propellants and explosives in aerospace, military, and civil applications. Explosives from inorganic reagents, though similar in the energy released per unit weight from conventional organic explosives, have the potential to release 3 to 5 times the energy per unit volume more than organic explosives.2Al+3CuO→Al2O3+3Cu  Equation 1
The reagents for thermite reactions are both solid materials which do not readily permit their mixing in a manner where a self propagating reaction is readily and consistently achieved. The use of such reagents as reactive powders was developed in the early 1960s, spawning what is known as Self-Propagating, High-Temperature Synthesis (SHS) where a wave of chemical reaction propagates from an ignition site over the bulk of the reactive mixture by layer-by-layer heat transfer. SHS reactions often require substantial preheating to self-propagate. Controlling the rate and manner in which their energy is released in these reactions is often difficult. Where very fine powders, whose mixtures are also referred to as metastable intermolecular composites, are used, thermite reactions are often defined as superthermite reactions as the nature of the small particles overcome some of the difficulties in achieving a readily initiated self-propagating reaction. Performance properties of such energetic materials are strongly dependent on particle size distribution, surface area of the constituents, and void volume within the mixtures. The general approach to improving such reactions between solid materials has been to increase the amount and nature of the interface between the solid reactants.
Drawing techniques have been used to achieve a large interface area between the two solid reactants. In these applications a relatively large metal rod is periodically drilled and filled with the metal oxide and drawn until the final material is in the form of a thin wire. This technique is known to have limitations with respect to the homogeneity of the mixture.
One approach to increasing the interface between solid reactants has been to been to use thin films of the materials in a laminate type. Success with this approach has required that films are prepared that have individual layer thickness in the range of microns to as small as angstroms. Such thicknesses have required methods such as vapor deposition. Unfortunately, vapor deposition techniques are generally impractical for the formation of large quantities of such materials due to the nature and expense of the process.
To accommodate techniques common for the fabrication of propellants and explosives, the use of powders has generally been chosen. In these applications homogeneous mixing is essential at the desired stoichiometry, which is not always achieved, as the mixing of two powders can be very inconsistent. With larger sized particles, such as 1 or more 1 μm in diameter, the amount of effective interface can be lower than desired and the initiation and propagation of reactions can suffer. Further complicating this approach is that commercially available nanoparticles, of significantly less than 1 μm in diameter, generally do not provide the quality of interface that is necessary as virtually all of these metal particles appropriate for thermite reactions form an oxide layer on their surface upon exposure to air. In the case of aluminum, the most commonly used metal for such systems, the oxide layers can be very thick relative to the diameter of the particles, and in the worst case can be almost exclusively aluminum oxide. This problem has led to the investigation of co-milling the metal with the metal oxide to give a homogeneous nanoparticulate mixture.
The milling of such mixtures has the advantage that it can begin with larger particles where the metals have a relatively small, generally insignificant, amount of oxide layer. However, co-milling processes tend to initiate the thermite reaction and do not permit the isolation in a manner that yields consistently viable thermite mixtures.